Tissue engineering is an emerging field combining both methods of the engineering and the principles of life sciences to understand the structural and functional relationships in normal and pathological mammalian tissues. The goal of tissue engineering is the development and ultimate application of biological substitutes to restore, maintain or improve tissue functions. Skalak, R. and Fox, C. F., "Tissue Engineering," Alan R. Liss Inc. N.Y. (1988).
One major component of these biological substitutes can be collagen or collagenous tissue. While major strides have been made in this field, and are continuing to be made, there is a constant need for improvement of both the biological substitutes and the processes for making and using them. There currently exists a need for a sterilization method that maintains the biological and physical properties of collagen and collagenous tissues used in tissue engineering. The search for sterilants to sterilize collagen and collagenous tissue has resulted in the use of a variety of different methods.
Physical sterilization methods have included heat, such as boiling, autoclaving, and microwaves. Heat, however, coagulates soft tissues. Additionally, heat at temperatures above 60.degree. to 65.degree. C. will denature collagen. Currently, gamma-irradiation is the preferred technique used to sterilize tissue, using between 0.5 and 2.5 megarads. A recent study has found, however, that collagen is damaged by gamma-irradiation at 1 megarad. Cheung et al., "The Effect of Gamma-Irradiation on Collagen Molecules Isolated Alpha Chains and Crosslinked Native Fibers," J. Biomedical Material Research, 24:581-590 (1990). These lower dose ranges are used since irradiation doses in excess of 2 megarads has a detrimental biological effect. Schnell et al., "The Influence of Ionizing Radiation on Various Collagen-Containing Medical Products," Radiosterilization of Medical Products, Paper SM92, International Atomic Energy Agency, Vienna (1967).
Known chemical sterilization methods produce chemical reactions that not only sterilize collagen, but also cross-link it. Collagen, cross-linked by chemical sterilization, results in a collagen tissue that is stiffer than uncross-linked collagen, remodels less well, and can evoke an immunological reaction. Thus, the chemical sterilants formaldehyde and glutaraldehyde cross-link collagen and reduce its capacity to remodel after implantation. Kato et al., Journal Biol. Jt. Surgery, 73:561 (1991).
Additionally, not all chemical sterilants are suitable for collagen sterilization. Ethyl and isopropyl alcohol are not sporicidal. Organic mercurials are toxic and have an injurious effect on bone osteoinductive protein. Beta-propiolactone is only effective for surface sterilization and, because of its carcinogenicity, it has been removed from the market. Ethylene oxide has been used successfully for sterilization of bone, but can dissolve soft tissue.
All of the sterilization techniques mentioned above have some drawbacks or are only partially effective for the sterilization of collagen. The ideal sterilant must be able to sterilize without altering the essential physical and biological nature of the collagen.
Peracetic acid is a known germicidal sterilant. In medical applications, aqueous or aqueous-ethanol peracetic acid solutions have been used typically to sterilize surfaces of instruments. Malchesky, P. S., "Peracetic Acid and Its Application to Medical Instrument Sterilization," Artificial Organs, 17:147-152 (1993). Sterilization of collagenous tissues by peracetic acid using these known techniques, however, is adversely affected by the swelling and dissolution of the collagen caused by the acidity of the solution.
Thus, there continues to be a need for a sterilant that can effectively sterilize collagen intended for use as a biological substitute in tissue engineering applications that maintains the biological and physical properties of the collagen or collagenous material.